Project Summary/Abstract: Cytokinesis is essential for development and survival of all organisms. Defects in cytokinesis cause aneuploidy and genomic instability, thereby contributing to serious diseases such as cancer, neuronal disorders, and anemia. Thus, mechanistic study of cytokinesis is important not only for understanding the basic principles of a fundamental process but also for designing new strategies to treat human diseases. Cytokinesis in animal and fungal cells requires spatiotemporally coordinated functions of a contractile actomyosin ring (AMR), targeted vesicle fusion, and localized extracellular matrix (ECM) remodeling. It is much more complex than previously appreciated. In this application, we will address three major unanswered questions regarding this fundamental process using both budding yeast and mammalian cell models, with the goal of dissecting deep mechanisms in yeast and exploring evolutionary conservation in mammalian cells. In Aim 1, we will determine the architecture of the AMR. Specifically, we will examine how myosin-II and its associated proteins such as actin and IQGAP are organized in the contractile ring from cells synchronized at cytokinesis using platinum-replica electron microscopy (PREM) coupled with immuno-gold labeling as well as super-resolution stochastic optical reconstruction microscopy (STORM). In Aim 2, we will test our hypothesis that myosin filament assembly is regulated by heavy chain phosphorylation as well as by trans-acting factors such as IQGAP using biochemical, genetic, quantitative live imaging, and other cutting- edge imaging methods as described above. In Aim 3, we will determine how the AMR guides exocytosis and ECM remodeling at the division site. Specifically, we will test our hypothesis that the tail of the yeast myosin-II positions and unloads vesicles from the transport machinery at the division site by interacting with the vesicle-associated guanine-nucleotide-exchange factor (GEF), Sec2, for the Rab GTPase Sec4. Then, the myosin-associated protein complex (Inn1, Hof1, and Cyk3) promotes vesicle fusion via the C2 domain of Inn1, and activates the cargo enzyme Chs2, a member of the glycosyltransferase family 2, for ECM remodeling (i.e. septum formation in yeast) via the transglutaminase-like domain of Cyk3. The discovery (Aim 1) and hypothesis-driven (Aims 2 and 3) research is expected to generate novel concepts and mechanisms of cytokinesis that are beyond specific model organisms.